Differential rotary drivers for tightening threaded fasteners having means at the threaded end of the bolt or pin to be drivingly engaged with a driver or the like have been well known though not extensively used. Such machines typically have two concentric output shafts which rotate concurrently in opposite directions. One output shaft is typically a central shaft and the other is circumferential, surrounding the central shaft. The central shaft is adapted to engage the bolt or pin of a fastener while the circumferential shaft engages the nut or collar. Such machines are shown, for example, in U.S. Pat. No. 2,928,302 issued in 1960 to Owen et al, entitled "MEANS FOR ACHIEVING A PREDETERMINED EXTENT OF LOADING IN TIGHTENING UP NUTS ON BOLTS AND STUDS"; in U.S. Pat. No. 3,041,902 issued in 1962 to Wing, entitled "MOTOR OPERATED HAND TOOL FOR SETTING FASTENERS"; and in U.S. Pat. No. 3,331,269 issued in 1967 to Sauter, entitled "DRIVING GUN".
All of the machines shown in those prior patents were portable, and the hand of the operator supported the housing or stator of a primary driver within which a power input shaft or rotor was drivingly rotated. In all of those machines both output shafts were coaxial to the power input shaft. One output shaft could be said to rotate in the clockwise direction while the other could be said to rotate in the counterclockwise direction. The clockwise output shaft would create a reaction torque to the operator of the machine in a counterclockwise direction and the counterclockwise output shaft would create a reaction torque to the operator in the opposite or clockwise direction.
It may have been a design objective of such machines to equalize those two reaction torques so that there would then be no net reaction torque experienced by the operator. This was clearly implied in the Sauter patent which stated at Col. 3, lines 56-60:
". . . these torques may be equal so that there is no torque upon the operator holding the driving gun 10. In the present gun there is a slight amount of such torque due to the speed reducing effects of sun gear 66, planet gears 74 and 76 and ring gear 84."
However, Sauter's machine failed to eliminate the reaction torque. Sauter's explanation of the problem was also wrong, because in the type of machine shown by Sauter it was both theoretically and practically impossible to eliminate the reaction torque imposed upon the hand of the operator. The machines described in the Owen et al patent and in the Wing patent also failed to eliminate reaction torque imposed upon the operator, and for the same reason.
Recent medical research has shown that operators of power drivers and the like, who experience reaction torque on a regular basis, are prone to chronic and serious ailments of the hand. Hence it is indeed important to eliminate this problem.
Another very desirable design objective for a differential rotary drive machine, but which the machines shown in the three patents described did not meet, is the establishment of optimum driving torques for the two output shafts.
Thus the present invention deals with eliminating the reaction torque experienced by the operator, and at the same time optimizing the driving torques of the two output shafts.